A catalyst used to promote a reaction is essential for a solid polymer electrolyte fuel cell. Platinum and platinum alloys have been mainly examined as catalysts that can be used in both hydrogen and oxygen electrodes. A large overvoltage is generated particularly in an oxygen electrode (cathode). In order to reduce such overvoltage, it would be possible to increase the amount of supported platinum or platinum alloy that is used as a catalyst. However, such increase in the catalyst amount is not effective enough to cause overvoltage reduction. On the other hand, the problem of cost increase along with an increase in the catalyst amount is further intensified. Thus, it has been a major objective to strike a balance between cost and catalyst performance.
As an aside, a complex of a macrocyclic compound containing metal such as porphyrin (PP), phthalocyanine (Pc), or tetraazaannulene (TAA) has been examined as a catalyst that has oxygen-reducing ability. The basic idea is that such complex of a metal macrocyclic compound has the capacity to adsorb oxygen molecules so that such capacity can be utilized and applied to a reaction for electrochemically reducing oxygen molecules. It would be possible to apply, as a catalyst, such nitrogen-containing-platinum-based complex having a PtN4 chelate structure in which platinum (Pt) is coordinated with 4 nitrogen atoms to a practical electrode.
For instance, WO2003/004156 filed by the present applicant discloses a platinum-free chelate catalyst containing metal porphyrin. In addition, in WO2003/004156, the chelate catalyst comprises a nitrogen-containing organic transition metal complex, which contains a transition metal that differs from the aforementioned transition metal, and a chalcogen component, in addition to at least one unsupported transition metal salt. It is described that when an unsupported transition metal salt is used as a filler during carbon matrix formation, a highly porous material is formed upon thermal decomposition of the salt due to the forming effect. Patent Document 1 discloses, in the Examples, a nitrogen-containing organic transition metal complex, in which cobalt tetramethoxyphenylporphyrin (CoTMPP) serving as metal porphyrin, sulfur serving as a chalcogen component, and an unsupported iron oxalate serving as transition metal salt were used. In such case, Fe contained in iron oxalate reacted with sulfur such that iron sulfide was formed, followed by acid elution. Thus, sulfur, which serves as a chalcogen component, is an essential component in WO2003/004156.